The present invention relates to an electroacoustic converter film used for, for example, an acoustic device such as a speaker or a microphone. More particularly, the present invention relates to an electroacoustic converter film in which leading out of wiring can be performed easily and reliably at the time of mounting the electroacoustic converter film as the speaker or the like.
In recent years, the research related to flexible displays using a flexible substrate such as plastic has been progressing.
As a substrate of such flexible displays, for example, JP 2000-338901 A discloses a flexible display substrate obtained by laminating a gas barrier layer or a transparent conductive layer on a transparent plastic film.
The flexible displays are superior to the conventional displays using a glass substrate in terms of the lightweightness, slimness, flexibility, and the like, and can be placed on a curved surface of a column and the like. Moreover, since flexible displays can be stored in a state of rolled, portability thereof does not deteriorate even the screen size thereof is large. Therefore, flexible displays are drawing attention for usage in apparatuses for displaying advertisement and the like or for usage in display apparatuses such as a personal digital assistance (PDA).
When these flexible displays are used as an image display apparatus-cum-sound generation apparatus such as a television receiver that reproduces images with sound, a speaker which is an acoustic apparatus for generating sound is required.
Regarding the shape of conventional speakers, generally, there are so-called cone-type speakers having a funnel shape, dome-type speakers having a spherical shape, and the like. However, if these speakers are built in the aforementioned flexible displays, lightweightness or flexibility which is an advantage of the flexible displays may be impaired. Moreover, if the speakers are installed externally, it is inconvenient to carry the displays, it is difficult to install the displays to a curved wall, and the external appearance may not be aesthetically pleasing.
Under these circumstances, for example, JP 2008-294493 A discloses that as a speaker, which can be integrated with a flexible display without impairing the lightweightness or flexibility, a sheet-like piezoelectric film having flexibility can be adopted.
The piezoelectric film is obtained by performing polarization processing on a uniaxially stretched polyvinylidene fluoride (PVDF) film at a high voltage, and has a property of expanding and contracting in response to an applied voltage.
In order to adopt the piezoelectric film as a speaker, the expansion and contraction movement performed along the film surface need to be converted into vibration of the film surface. The expansion and contraction movement can be converted into vibration by holding the piezoelectric film in a state of curved, and in this manner, the piezoelectric film can be caused to function as a speaker.
Incidentally, it is well known that a lowest resonance frequency f0 of a speaker diaphragm is calculated by the following equation. In the equation, s is stiffness of a vibration system, and m is a mass.
Lowest Resonance Frequency:
      f    0    =            1              2        ⁢        π              ⁢                  s        m            
At this time, as a degree of bending of the piezoelectric film, that is, as a radius of curvature of a bending portion increases, the mechanical stiffness s decreases, hence the lowest resonance frequency f0 is reduced. That is, the sound quality (volume and frequency characteristics) of the speaker varies with the radius of curvature of the piezoelectric film.
In order to solve the above problem, in JP 2008-294493 A, the speaker has a sensor for measuring a degree of bending of the piezoelectric film, and according to the degree of bending of the piezoelectric film, sound quality is corrected by means of increasing or decreasing the amplitude by a predetermined amount for each frequency band of the audio signals, whereby stabilized sound can be output.
When a flexible display, which is integrated with a speaker formed of a piezoelectric film and has a rectangular shape in a plan view, is gripped in a state of gently bent just like documents such as newspaper or a magazine as a portable apparatus and is used in a state where the length and breadth has been switched for screen display, it is preferable for the image display surface to be able to bend not only in the longitudinal direction but also in the horizontal direction.
However, since the piezoelectric film formed of uniaxially stretched PVDF has in-plane anisotropy as the piezoelectric characteristic, the sound quality varies significantly with the bending direction even if the curvature is the same.
In addition, since loss tangent of PVDF is smaller than that of the ordinary speaker diaphragm such as cone paper, resonance thereof easily becomes strong, and frequency characteristics thereof shows great fluctuation of frequency. Accordingly, when the lowest resonance frequency f0 varies with the change in the curvature, the sound quality also changes to a large extent.
As described above, due to the problems intrinsic to PVDF, it is difficult for the sound quality correction means disclosed in the aforementioned JP 2008-294493 A to reproduce stabilized sound.
Meanwhile, as an example of sheet-like flexible piezoelectric materials which do not have in-plane anisotropy as a piezoelectric characteristic, there is a polymeric composite piezoelectric body obtained by dispersing a piezoelectric ceramic in a polymer matrix.
In the polymeric composite piezoelectric body, the piezoelectric ceramic is hard while the polymer matrix is soft. Therefore, there is a possibility that energy may be absorbed before vibration of the piezoelectric ceramic is transmitted to the entire piezoelectric body. This is called a transmission efficiency of dynamic vibrational energy. In order to improve the transmission efficiency, the polymeric composite piezoelectric body needs to be hardened, and for doing this, the volumetric proportion of the piezoelectric ceramic added to the matrix needs to be at least 40% to 50% or higher.
For example, “Toyoki KITAYAMA, Showa 46′ Journal of National Convention of The Institute of Electronics, Information and Communication Engineers, 366 (1971)” discloses that a polymeric composite piezoelectric body, which is obtained by mixing powder of PZT ceramic as a piezoelectric body with PVDF by means of solvent casting or hot kneading, establishes both the flexibility of PVDF and a high degree of piezoelectric characteristics of PZT ceramic to some extent.
However, if the proportion of the PZT ceramic is increased to improve the piezoelectric characteristics, that is, the transmission efficiency, this results in a mechanical defect that the piezoelectric body becomes hard and brittle.
In order to solve such a problem, for example, “Seiichi SHIRAI, Hiroaki NOMURA, Juro OGA, Takeshi YAMADA, and Nobuki OGUCHI, Research Report of The Institute of Electronics, Information and Communication Engineers, 24, 15 (1980)” discloses an attempt at maintaining flexibility by adding fluororubber to PVDF.
From the viewpoint of flexibility, this method produces a certain effect. However, generally, rubber has a Young's modulus of 1 MPa to 10 MPa which is an extremely small value. Therefore, the addition of the rubber decreases the hardness of the polymeric composite piezoelectric body, and as a result, the transmission efficiency of vibrational energy also decreases.
As described above, when the conventional polymeric composite piezoelectric body is used as a speaker diaphragm, if an attempt at imparting flexibility to the speaker is made, the energy efficiency unavoidably decreases. Therefore, the speaker cannot produce a sufficient performance as a speaker for a flexible display.
From the above, it is preferable for the polymeric composite piezoelectric body used as a speaker for flexible displays to satisfy the following requirements.
(i) Flexibility
For example, when a flexible display is gripped in a state of gently bent just like documents such as newspaper or a magazine as a portable apparatus, the display constantly and externally experiences severe bending deformation which is caused relatively slow and of which the frequency is several Hz or lower. At this time, if the polymeric composite piezoelectric body is hard, a bending stress as great as the hardness is caused. Consequently, the interface between the polymer matrix and particles of the piezoelectric body may crack and be broken in the end. Therefore, the polymeric composite piezoelectric body is required to have an appropriate degree of softness. If the strain energy can be caused to diffuse outside in the form of heat, the stress can be relaxed. Accordingly, the polymeric composite piezoelectric body is required to have an appropriately large loss tangent.
(ii) Sound Quality
The speaker vibrates particles of the piezoelectric body at a frequency in an audio band of 20 Hz to 20 kHz, and causes the entire diaphragm (polymeric composite piezoelectric body) to vibrate as a whole by the vibrational energy, thereby reproducing sound. Therefore, in order to increase the transmission efficiency of the vibrational energy, the polymeric composite piezoelectric body is required to have an appropriate degree of hardness. If the speaker has smooth frequency characteristics, when the lowest resonance frequency f0 varies with the change in curvature, the sound quality changes in a small extent. Consequently, the loss tangent of the polymeric composite piezoelectric body needs to be appropriately great.
To summarize, the polymeric composite piezoelectric body used as a speaker for flexible displays is required to exhibit hardness with respect to vibration of 20 Hz to 20 kHz while exhibiting softness with respect to vibration of a frequency of several Hz or lower. Furthermore, the loss tangent of the polymeric composite piezoelectric body is required to be appropriately great with respect to vibration at all frequencies of 20 kHz or lower.
In order to satisfy the above requirements, the inventors of the present invention focused on a viscoelastic material which has a large frequency dispersion in a storage modulus (E′) and also has a peak value of a loss tangent (Tan δ) at a temperature around normal temperature, and conducted thorough examination to apply the viscoelastic material to a matrix material. As a result, by using the viscoelastic material, the inventors have devised an electroacoustic converter film formed of a polymeric composite piezoelectric body which exhibits hardness with respect to vibration at a frequency of 20 Hz to 20 kHz while exhibiting softness with respect to vibration at a frequency of several Hz or lower, and has an appropriate loss tangent with respect to vibration at all frequencies of 20 kHz or lower.